From the Book

Legacy Ethernet Technologies

Ethernet is best understood by first considering the two early Ethernet specifications—10BASE5 and 10BASE2. With these two specifications, the network engineer installs a series of coaxial cables connecting each device on the Ethernet network, as shown in Figure 28-2.

The series of cables creates an electrical circuit, called a bus, which is shared among all devices on the Ethernet. When a computer wants to send some bits to another computer on the bus, it sends an electrical signal, and the electricity propagates to all devices on the Ethernet.

With the change of media to UTP and the introduction of the first hubs, Ethernet physical topologies migrated to a star as shown in Figure 28-3.

Regardless of the change in the physical topology from a bus to a star, hubs logically operate similar to a traditional bus topology and require the use of CSMA/CD.

CSMA/CD

Because Ethernet is a shared media where every device has the right to send at any time, it also defines a specification for how to ensure that only one device sends traffic at a time. The CSMA/CD algorithm defines how the Ethernet logical bus is accessed.

CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:

A device with a frame to send listens until the Ethernet is not busy.

When the Ethernet is not busy, the sender(s) begin(s) sending the frame.

The sender(s) listen(s) to make sure that no collision occurred.

If a collision occurs, the devices that had been sending a frame each send a jamming signal to ensure that all stations recognize the collision.

After the jamming is complete, each sender randomizes a timer and waits that long before trying to resend the collided frame.

When each random timer expires, the process starts again from the beginning.

When CSMA/CD is in effect, it also means that a device’s network interface card (NIC) is operating in half-duplex mode—either sending or receiving frames. CSMA/CD is disabled when a NIC autodetects that it can operate in—or is manually configured to operate in—full duplex mode. In full duplex mode, a NIC can send and receive simultaneously.

Legacy Ethernet Summary

Today, you might occasionally use LAN hubs, but you will more likely use switches instead of hubs. However, keep in mind the following key points about the history of Ethernet:

The original Ethernet LANs created an electrical bus to which all devices connected.

10BASE2 and 10BASE5 repeaters extended the length of LANs by cleaning up the electrical signal and repeating it—a Layer 1 function—but without interpreting the meaning of the electrical signal.

Hubs are repeaters that provide a centralized connection point for UTP cabling—but they still create a single electrical bus, shared by the various devices, just like 10BASE5 and 10BASE2.

Because collisions could occur in any of these cases, Ethernet defines the CSMA/CD algorithm, which tells devices how to both avoid collisions and take action when collisions do occur.